KVM: X86: Reset vcpu->arch.cpuid_nent to 0 if SET_CPUID* fails

Current implementation keeps userspace input of CPUID configuration and
cpuid->nent even if kvm_update_cpuid() fails. Reset vcpu->arch.cpuid_nent
to 0 for the case of failure as a simple fix.

Besides, update the doc to explicitly state that if IOCTL SET_CPUID*
fail KVM gives no gurantee that previous valid CPUID configuration is
kept.
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-Id: <20200708065054.19713-2-xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -669,6 +669,10 @@ MSRs that have been set successfully.
 Defines the vcpu responses to the cpuid instruction.  Applications
 should use the KVM_SET_CPUID2 ioctl if available.
 
+Note, when this IOCTL fails, KVM gives no guarantees that previous valid CPUID
+configuration (if there is) is not corrupted. Userspace can get a copy of the
+resulting CPUID configuration through KVM_GET_CPUID2 in case.
+
 ::
 
   struct kvm_cpuid_entry {

arch/x86/kvm/cpuid.c

@@ -209,6 +209,8 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
 	kvm_apic_set_version(vcpu);
 	kvm_x86_ops.cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
+	if (r)
+		vcpu->arch.cpuid_nent = 0;
 
 	kvfree(cpuid_entries);
 out:
@@ -232,6 +234,8 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
 	kvm_apic_set_version(vcpu);
 	kvm_x86_ops.cpuid_update(vcpu);
 	r = kvm_update_cpuid(vcpu);
+	if (r)
+		vcpu->arch.cpuid_nent = 0;
 out:
 	return r;
 }